Microwave filters



21, 1956 H. F. ENGELMANN 2,760,169

MICROWAVE FILTERS Filed Aug. 1, 1951 2 Sheets-Sheet 1 6 z ifi A: 7 v 9 Q7/ 1 2 J y 5 9 5' l9 /5 20 INVENTOR HERBERT E ENGELMANN Aug. 21,1956 H. F. ENGELMANN MICROWAVE FILTERS 2 Sheets-Sheet 2 Filed Aug. l, 1951 INVENTOR HERBERT F. E/VGELMANN ATTORNEY United States Patent MICROWAVE FILTERS Application August 1, 1951, Serial No. 239,795, 4 Claims. on. 3-33-73 This invention relates to radio frequency filters and more particularly to filters for application to microwave frequencies.

Microwave filters have heretofore required expensive conductor systems employing waveguides and coaxial lines. As technical development reached into higher frequencies, the precision requirements of these filters have become more exacting thereby rendering difficult the manufacture of satisfactory filters for microwave frequencies. One of the objects of this invention is to provide a simplified form of microwave filter which does not require the precision and exactness of microwave filters heretofore believed necessary.

Another object of the invention is to provide a microwave filter that may employ as a part of the filter a wall of the chassis or other apparatus associated with the filter.

Still another object of the invention is to provide a microwave filter which is of a character readily adapted for the use of printed circuit techniques.

A further object is to provide microwave filter structures having low loss characteristics.

One of the features of the invention is. its utilization of a basic principle present in a theoretically perfect parallel line transmission system. The principles involved are disclosed in the copending applications of D. D. Grieg and H. F. Engelmann, Serial No. 227,896, filed May 23, 1951, and Serial No. 234,503, filed June 30, 1951, now Patent No. 2,721,312. The present invention utilizes this theoretically perfect parallel line principle without requiring the exact identity and spacing of two parallel, conductors. The filters are composed of two closely spaced conductors with one of the conductors, hereinafter sometimes referred to as the ground conductor, which is wider than the other conductor, hereinafter sometimes referred to as the line conductor, so as to produce effect an image of the line conductor. Another feature is the shaping of the ground conductor so as to trough the line conductor portions of the filter and thereby minimize radiating loss.

The above-mentioned and other features and objects of this invention will be best understood by reference to the following description taken in conjunction with the. accompanying drawings, wherein:

Fig. 1 is a plan view of a filter useful in explaining the principles of the invention;

Fig. 2 is a cross-sect=ional view taken along line 2-2 of Fig. 1;

Fig. 3 is a plan view of a difierent form of filter;

Fig. 4 is a plan view of still another form of filter;

Fig. 5 is a cross-sectional view taken along line 55 of Fig. 4;

Fig. 6 is a plan view of :a multisection filter;

Fig. 7 is a cross-sectional view taken 'along line 7-7 of Fig. 6; and

Fig, 8 is a plan view of still another multiple section filter.

Referring to Figs. 1 and 2 of the drawing, a filter is shown in accordance with the principles of this invention as comprising a first or line conductor 1 and a second or ground conductor 2 disposed in close parallel, spaced relation, the conductor 1 being supported by a layer of dielectric material 3 covering the conductor 2. The conductor 2 preferably comprises a flat conductor presenting a plane surface above which the line conductor 1 is disposed. The plane or ground conductor 2 is selected wide as compared to the line conductor 1 so that the electric field is concentrated between the opposite surface of the conductors. The conductor 2 may comprise a wall of a chassis or a panel, or it may comprise a metallic coating on the layer of dielectric 3. The conductor 1 may comprise conductive strips :applied by a die-stamping operation, or it may comprise conductive material applied by any one of a number circuit printed techniques.

The conductor 1 is selected in the embodiment of Fig. 1 substantially one-half wavelength, or a multiple thereof, of the mid-frequency of the microwave energy to be passed by the filter. Both ends 4 and 5 of the conductor strip 1 are connected directly to the ground conductor 2 as indicated at 6 in Fig. 2. At resonant condition the currents at the ends 4 and 5 are of maximum value, while the voltage is a maximum midway along the conductor strip 1. An input connection 7 which may comprise a strip of conductive material supported on the layer of dielectric 3, is connected at a point 8 a selected distance from the end 4. Likewise, an output connection 9 is connected at a point 10. a similar selected distance from the end 5. The points of connection 8 and 10, however, may be considerably varied depending upon the impedance match re* quired in the associated circuitry. The points 8 and 10, for example, may be chosen to provide any impedance ratio desired for the input and output conductors.

While the length of the conductor 1 is selected substantially one-half wavelength of the frequency to be passed, it should be noted, however, that the filter will also pass frequencies which are integral multiples of this frequency.

Referring to Fig. 3, a modified form of filter is shown, wherein the input connection 11 and output connection 12 are coupled to the conductor 1 by capacity. The conductor 11, which may comprise a line conductor supported by insulation 3, is terminated in close, spaced relation to the line conductor 1 on one side, and the conductor of the output connection 12 is likewise disposed in close, spaced relation to the opposite side of the line conductor 1. This capacitive coupling arrangement may be located in the mid-point along the conductor 1 where the voltage is a maximum, or they may be located at other selected points, opposed or staggered, along the conductor 1. The magnitude of the capacity for coupling is inversely proportional to the spacing separating the coimections 11 and 12 from the conductor 1 and hence may also be altered by varying this spacing.

In Figs. 4 and 5 a similar filter arrangement is shown with the same type of capacitive coupling between the input and output connections 11, 12 and the conductive strip 13, The conductive strip 13, however, is selected.

substantially one-quarter wavelength long and has one end 14 grounded to the plane conductor 15. The other end 16 is open. In this embodiment the ground or plane conductor 15 is shown to have been grooved or etched to provide a trough 17, Fig. 5, in which a layer of dielectric material 18; is disposed so. as. to. support the conductive strip 13 and the input and output connections 11 and 12. By this construction the side walls 19 and 20 of the trough serve to minimize radiation losses. The electric field is substantially entirely concentrated between the surface of the strip 13 and the bottom and side wall portions of the trough 17.

Figs. 6 and 7 show a filter which comprises a plurality of filter sections similar to the embodiment of Fig. 3 coupled together in series relation. Two end filter sections 24 and 25 are shown, each section comprising a conduc- Patented Aug, 21, 1956.

tive strip 1 grounded at its ends 4 and 5 the same as shown in Fig. 3. The grounded connection 6, however, is shown to be disposed in an enlarged opening 27 in the layer of dielectric 3 although the openings 27 need be only large enough to accommodate connection 6. The input connection 28 is shown coupled by capacity by the spacing 29 similarly as in Fig. 3, and the output connection 30 is likewise coupled by capacity spacing to the conductor strip 1 of section 25. Adjacent sections are similarly coupled by a conductive strip 31 To minimize radiation losses laterally with respect to the conductor strips 1 and 31, upright conductive ridges 32 and 33 are disposed in close, spaced relation with the sides of the conductive strips 1, 31 and also with respect to the input and output connections 28 and 30. These conductive ridges are connected to and made a part of the ground conductor 2. These ridges extend, preferably, at least as far from the plane surface of the conductor 2 as the conductor strip 1. In Fig. 7 these ridge conductors are shown to extend even further so as to substantially completely shield in lateral directions the spacing between the conductor strips and the ground conductor 2. At the input and output connections these conductive ridges are flared outwardly in horn fashion as indicated at 34 and 35 with respect to the line conductor, thereby minimizing perturbation of the microwave energy at the input and output connections.

The series filter shown in Fig. 8 illustrates 'a series connection for the type of filter coupling shown in Fig. 1. In the embodiment shown, a number of identical filter sections are indicated, the input section 36 having the input connection 37 directly connected to the conductor strip 1 thereof as indicated at 38. Likewise, the output section 39 is provided with an output connection 40 connected to the conductor 1 thereof as indicated at 41. Adjacent sections are similarly connected directly as indicated by connecting links 42 and 43. To minimize lateral radiation losses, conductive ridges 44 and 45 are disposed along the line conductor portions which comprise the input connections, filter sections, and connecting strips substantially as shown, the conductive ridges being flared outwardly at the input and output connections. By using the side ridges radiation loss is maintained low, thereby presenting filter sections which possess high selectivity. While the filter sections of Figs. 6 and 8 are shown to be substantially equal in length, it will be clear that filter sections may be selected of different lengths, whereby selectivity may be obtained over a wide range of frequencies. In the multisection resonant filter the length of the filter sections, for example, may be chosen equal to half wavelengths of a number of different frequencies of a desired pass band. It should also be understood that the coupling between adjacent filter sections may either be by the direct coupling method, as indicated in Figs. 1 and 6, or by spaced capacitive coupling, as indicated in Figs. 3, 4 and 8.

While I have described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made by way of example only and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A microwave filter comprising a layer of solid dielectric material, a conductor having a plane surface supported on one face of said layer, a line conductor supported on the other face of said layer in closely spaced parallel relation to said plane surface, said line conductor being of a length to form in conjunction with said plane surface a resonant section for the frequency energy to be passed, said plane surface being of dimensions greater than said line conductor so that said line conductor is entirely Within the confines of said plane surface and the spacing therebetween being such that at said frequency substantially the entire electric field distribution is concentrated between said line conductor and said plane surface in a manner substantially the same as the field distribution between one conductor and the neutral plane of a two-conductor parallel system, and coupling conductors supported on said other face of said layer of dielectric above said plane surface with the ends thereof coupled to said line conductor at points spaced from the ends of said line conductor.

2. A microwave filter according to claim 1 further including means conductively connecting at least one end of said line conductor to said plane surface.

3. A microwave filter according to claim 1 wherein said coupling conductors have the ends thereof capacitively spaced at said points from said line conductor.

4. A microwave filter comprising a layer of solid dielectric material, a conductor having a plane surf-ace supported on one face of said layer, a line conductor of ribbonlike form supported on the other face of said layer in closely spaced parallel relation to said plane surface, said line conductor being of a length to form in conjunction with said plane surface a resonant section for the frequency energy to be passed, said plane surface being of lateral dimensions greater than said line conductor so that said line conductor is entirely within the confines of said plane surface and the spacing therebetween being such that at said frequency substantially the entire electric field distribution is concentrated between said line conductor and said plane surface in a manner substantially the same as the field distribution between one conductor and the neutral plane of a two-conductor parallel system, coupling conductors supported on said other face of said layer of dielectric above said plane surface with the ends thereof coupled to said line conductor, said plane conductor having upright wall portions disposed laterally of said line conductor and said coupling conductors, said wall portions being of a height corresponding substantially to the spacing between said line conductor and said planar surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,149,387 Brown Mar. 7, 1939 2,159,648 Alford May 23, 1939 2,196,272 Peterson Apr. 9, 1940 2,203,481 Zottu June 4, 1940 2,274,346 Roosenstein Feb. 24, 1942 2,410,656 Herold Nov. 5, 1946 2,411,555 Rogers Nov. 26, 1946 2,432,057 Wheeler Dec. 2, 1947 2,513,761 Tyson July 4, 1950 2,527,664 Wheeler Oct. 31, 1950 2,583,854 Kehbel Jan. 29, 1952 2,611,822 Bliss Sept. 23, 1952 2,624,801 Richards Ian. 6, 1953 FOREIGN PATENTS 601,514 Great Britain May 7, 1948 

